Growing interest in renewable energies due to shrinking reserves of fossil fuels and climate change concerns have led to extensive research towards gaseous and liquid fuels production from renewable energy resources such as biomass and waste. Energy generation from municipal and industrial wastes such as wastewater sludge is also environmental friendly way to deal with large volume of waste disposal with the additional advantage of eliminating part of the indirect greenhouse gas emissions from energy crops-derived biofuels [1]. Municipal and industrial wastewater treatment plants generate a large volume of waste activated sludge (WAS) as a result of biological treatment of the wastewater. This produced sludge poses a threat to the environment and needs further treatment prior to disposal or incineration [2]. Sludge handling and management costs may be as high as 25-50% of the total cost of the wastewater treatment process [3]. Recently there has been a rising interest in developing more environmentally friendly processes to reduce the volume of the sludge for disposal and replacing the conventional sludge disposal methods such as landfill disposal and incineration by converting sludge into bio-energy.
Improved management of biosolids has been identified as a targeted research area in the Canada-wide Strategy for the Management of Municipal Wastewater Effluent. Endorsed by the Canadian Council of Ministers of the Environment, it is aligned with the rising interest in environmentally friendly processes to reduce the volume of sludge for disposal and find methods of utilizing the matter to produce bioenergy and more valuable products.
There are numerous drop-in biofuel technologies under development globally. The most advanced processes operating at commercial scale generally require relatively clean, dry, and homogenous feedstocks such as virgin vegetable oils, algal oil, waste animal fats, and used cooking oil. Neste Oil is a global leader in hydroprocessing vegetable oils to hydrocarbon liquid fuels, with commercial scale plants operating in Finland, the Netherlands, and Singapore for a total production capacity of approximately 2 million tonnes per year. Pyrolysis technology is also being commercialized using woody biomass as feedstock. The initial product, often referred to as pyrolysis oil or bio-oil, can be used as lower grade heating oil or can be upgraded to industry standard hydrocarbon liquid fuels. KiOR has a commercial plant operating in Mississippi producing 40,000 tonnes per year of gasoline, diesel, and heating oil. Envergent Technologies, a Honeywell company, uses Ensyn's rapid thermal process (RTP®) technology also for conversion of woody biomass to pyrolysis oil. BTG-BTL company in the Netherland has also developed and commercialized BTL (biomass-to-liquid) pyrolysis process that converts up to 70 wt. % of the biomass feedstock into bio-oil and the remaining part into char and gas. (See https://www.btg-btl.com/en/technology).
Hydrothermal liquefaction (HTL) is a thermo-chemical depolymerization process used to convert wet biomass into crude—like oil—sometimes referred to as bio-oil or biocrude under moderate temperature and high pressure developed to produce energy from biomass in the presence of water to avoid the energy-intensive prior drying [4]. It is a promising technology for converting waste biomass with high water content into value-added products, mainly bio-crude oil and solid residue (bio-char) in the absence of oxygen at 150-450° C. and pressure up to 25-30 MPa [5]. It eliminates the need of a costly de-watering/drying process that is otherwise required in other thermal/thermo-chemical processes. The remarkable properties of water such as low dielectric constant and high ionic product, play important roles as a solvent in liquefaction. The process can be made self-sufficient in energy using a part of the produced oil and char to provide heat for the HTL process.
The reaction typically uses homogeneous and/or heterogeneous catalysts to improve the quality of the produced products and yields. The carbon and hydrogen of the organic starting material, such as, but not limited to, biomass, low-ranked coals (lignite) and peat are thermo-chemically converted into hydrophobic compounds with low viscosity and high solubility. Depending on the processing conditions, the resulting fuel can be used as is for heavy engines such as rail or marine based engines, or the output may be upgraded to transportation fuels, including jet-fuel, diesel and regular gasoline.
HTL technology offers several advantages to the emerging fast pyrolysis process. While the process operating pressure for HTL is higher, the lower temperature and the ability to utilize wet sludge are the critical advantages. It has been found to be cost-effective compared to incineration [6] and can achieve additional benefit of pathogen reduction meeting the stringent regulation on sludge land applications. Further, the quality of the produced bio-oil is higher, with lower water content (5%), lower oxygen content (20-30%), and higher energy content or heating value (30-35 MJ/kg). By utilizing wet organic waste solids, our HTL technology would represent a significant advancement to the biofuels industry mainly through the ability to utilize readily available high moisture organic waste.
Currently there is only a single sludge-to-oil technology established or under development for energy recovery from wastewater sludge based on hydrolysis and hydrothermal treatment. An early study of sewage sludge liquefaction was performed by Kranich and Eralp [7]. Sewage sludge was converted to oil at different reaction temperatures in the presence of hydrogen as a reducing gas and catalysts such as Na2CO3, NiCO3, and Na2MnO4. The oil yields were less than 20 wt % with water as the reaction medium [7], [8]. A pilot scale study was carried out by Molton et al. where primary and undigested sludge with 20% total solids (TS) were heated at 300° C. and 10 MPa pressure in a continuous reactor with 30 L/h flow rate and hydraulic retention time of 90 minutes. The technology was patented as sludge-to-oil reaction system (STORS) with oil yields ranging from 10-20 wt % and char from 5-30 wt % [5], [6]. It was commercialized by ThermoEnergy Company in 2005; however, there is currently no full-scale installation in operation.
Another competitive process for sludge processing is anaerobic digestion and biogas production and there are two commercial processes in operation. The Cambi process consists of three vessels (a pulping vessel, hydrolysis reactor, and a flash tank) and treats sludge under pressure at temperatures between 160-180° C. Cambi installations are now operating in Norway, Denmark, England, Ireland, Scotland, and Poland. The technology is relatively complex: solids from wastewater treatment must be dewatered to 16% dry solids prior to the process and a medium-pressure steam supply is required. Reports of odor problems have been associated with the process [9].
The BioThelys process is used to treat sludge with a solids concentration higher than 10% and operates at 150-180° C. and 8-10 bars pressure. Two full-scale facilities have been operating in France since 1998. Like the Cambi process, the BioThelys process may also be subject to odor concerns [9].